In a TDMA communication system, a number of transmitters can transmit on the same frequency channel, but at different times. A remote receiver for receiving a particular transmitted signal knows beforehand at what time the transmitted signal will occur and receives only during that time. The use of TDMA makes very efficient use of the frequency spectrum since multiple users may use the same frequency channel at the same time without interfering with one another.
FIG. 1 illustrates one type of TDMA system where handheld wireless telephones 10 and 11 share a same frequency channel while transmitting to and receiving from a nearby higher power base station 12. All communications made by telephones 10 and 11 are routed through base station 12 as long as telephones 10 and 11 are in the vicinity of base station 12. The various base stations in the TDMA system are connected to the public telephone lines (not shown).
Additional wireless telephones 13 and 14 are also shown in FIG. 1. Telephone 13 routes its communications through its closest base station 15, and telephone 14 routes its communications through its closest base station 16. The various base stations 12, 15, and 16 convert the received TDMA signals from the handheld telephones 10, 11, 13 and 14 to conventional analog POTS, BRI, or PRI signals for transmission over the telephone lines. Similarly, the base stations 12, 15, and 16 convert the received information on the telephone lines to TDMA signals for transmission to the handheld telephones 10, 11, 13, and 14. Thus, all of the handheld telephones 10, 11, 13 and 14 can communicate with another telephone inside or outside of the TDMA system.
The base stations 12, 15, and 16 receive or transmit time-division multiplexed signals simultaneously. To avoid co-channel interference between telephones using neighboring base stations, neighboring base stations generally operate on different frequency channels, where the frequency channel is automatically selected to minimize co-channel interference. Such a TDMA system may be the Personal Handy Phone System, common in Japan, whose requirements are described in the RCR Standard-28, incorporated herein by reference. As an alternate embodiment, such a TDMA system may be the European ETSI DECT standard, also incorporated herein by reference. Another such alternate embodiment may be the North American PACS standard. Further, slow frequency hopping systems, compliant with CFR Title 47, part 15, and intended for the U.S. ISM-bands, may be derived from the aforementioned formal standards. The operation of such TDMA systems are well known.
In a TDMA system, each wireless telephone 10, 11, 13, and 14, when active, is allocated certain time slots within which it may transmit a bursted signal or receive a bursted signal. FIG. 2 illustrates a frame 17 containing slots 0 through 7, where frame 17 is repeated on a single frequency channel. Other TDMA systems may use 24, 32, or other number of slots in a frame. The period of frame 17 may be, for example, 5 milliseconds. Assuming wireless telephones 10 and 11 in FIG. 1 are being actively used at the same time, telephones 10 and 11 may be allocated slots 0 and 1, respectively, for transmitting bursted signals to base station 12, while allocated slots 4 and 5, respectively, for receiving bursted signals from base station 12. Two additional telephones (e.g., 13 and 14) entering the vicinity of base station 12 may use the available slots 2, 3, 6, and 7 in a similar manner. The amount of information stored in each telephone 10, 11, 13, and 14 during a frame period is transmitted in a burst within a single slot.
A sample protocol 18 for a slot is also shown in FIG. 2, where protocol 18 dictates the information required to be transmitted during a single slot. Protocol 18 may consist of a ramp-up field 20, a start symbol field 21, a clock recovery field 22, a slot sync (or Unique Word) field 23, a data field 24, a CRC field 25 (for error correction and verification), and a guard band field 26. The lengths and types of fields in a protocol vary depending on the mode of the transceiver (e.g., registration mode, paging mode, call set up mode, transmit/receive mode, etc.). While in the traffic mode, where voice is to be transmitted, data field 24 contains audio data. Data field 24 is referred to as the traffic channel or TCH.
In one embodiment, the bit rate of the transmitted bits in a frame 17 is approximately 384K bits per second, and the modulation technique is .pi./4DQPSK. Therefore, the corresponding symbol rate is 192K symbols per second.
As the wireless telephones are transported in their communication mode to different areas handled by different base stations, there must be a non-detectable handover to another base station. Such a handover may require the telephone to transmit/receive in a different slot pair within a frame whether on the same frequency channel or a different frequency channel. To achieve such a handover and to maximize the use of the channel, each base station and wireless telephone transmits/receives slots and frames in synchronization so as to align the various slot times within a frame throughout the TDMA system.
Typically, with a hard-wired star or an ad-hoc RF base station synchronization topology, the various base stations remain in-sync by detecting an edge of another base station's signal and adjusting its local bit/slot pointer. Such a clock de-skewing technique has been known to suffer from coupling-induced EMI and other drawbacks. What is needed is a more robust synchronization technique for a TDMA communication system.